Why do teachers get paid at the rate they do?

Teachers educate the future. Everyone has had a teacher at some stage in their life. Great teachers can really make a difference in a child's life. I understand that there are 'poor' performing teachers out there but these people are found in every profession. This however, should not be used as an excuse as to why teachers are paid so little. People also have a huge misconception about the amount of work teachers actually do. It is not a 8:30-3:00 job- you never switch off. SO why do teachers get paid at the rate they do? Thank you!